v0.21.0 foundation: design-token module (#31)

Establish src/styles/tokens.css — the single source of truth for color,
type, spacing, radius, elevation, and motion. Imported first in main.jsx.
Sweep subagents map literal values to these tokens; no appearance change
is intended beyond consolidating near-duplicate grays (operator reviews
before deploy).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/main.jsx

@@ -2,6 +2,7 @@ import React from 'react'
 import ReactDOM from 'react-dom/client'
 import { BrowserRouter } from 'react-router-dom'
 import App from './App.jsx'
+import './styles/tokens.css'
 import './index.css'
 
 ReactDOM.createRoot(document.getElementById('root')).render(